You can use the Opera Developer builds if you want. There are 64-bit versions available. See http://www.opera.com/blogs/desktop/. You can think of them like dev/canary chrome builds. As in, they might be stable, but might not.

2016.1013.1234.CDT Thursday re 64-bit Opera
Until version 41 there will be no (new skul version) 64-bit Opera (new skul = shorthand for Opera based on Chromium Open Source Code - which was about version 13 or maybe 20 -- any historical data would help -- or look in the chrono dates for versions iyi [If You're Interested)/

64-bit original Opera was released originally as 12.17.1863 or 1873, followed shortly by a security update to aOpera or alte Opera or oOpera version 12.18.1873. Unfortunately I no longer know where that version/64-bit can be found. One of my personal 'likes' of the aOpera 12.18.1873 is that it is not a memory hog as so many other current browsers are -- it opens a main "opera.exe" process, and an "opera-plugin container" process; if there is a crash a third process "opera-crashreporter.exe" [keyboarding these names off the top of my head so they may not literally be accurate]; the other feature of note is aOpera was available as a "portableApp" and as Opera USB versions -- do NOT know if that is true any longer. Perhaps the most unusual feature is to use a keyboard shortcut to minimize aOpera 12.18.1873 to the systray as an systray icon with a right-click or context menu. And yes the a-opera.exe process does grow in size so the argument can be made it is not unlike the new Opera 40, Vivaldi 1.4 (I run the 64-bit version in all cases except for new Opera 40.0.2308.81), and also the 64-bit versions of Microsoft Office 2016/m*OFFice 2016/Office 365 Personal Edition.

Note Bene: * - using "m" prefix before an application or program designates the mobile or cellphone version of that, the app rather than the application.

The bit rate of an application and its security have nothing in common. All being 64-bit means is that it can access more memory at a time vs a 32-bit application. While Google did re-write their renderer to make use of 64-bit memory space, and that rewrite brought some improvements, it was not because it was 64-bit. It was because they were learning from past experience and incorporating new technologies.